1. Field of the Invention
The present invention relates generally to the field of ink compositions or to the field of oil based compositions usable in lithography, and more particularly to a lithographic ink composition in which its relationship with aqueous solutions, including its solubility, can be selectively controlled. The compositions of the present invention are unaffected by water, and thus generally water insoluble, at certain selected generally acidic pH levels and water washable at certain other selected generally alkaline pH levels. The present invention also relates to methods of making and using such compositions.
2. Summary of the Prior Art
A wide variety of printing processes currently exist in the art. Although it is contemplated that the ink composition and method of the present invention may have applicability to many of these prior processes, it has particular applicability to a lithographic printing process commonly referred to as lithography. Lithography is a method of printing which relies on differences in solubility and surface wetability between an oil based component and an aqueous or aqueous based component to effectively transfer the printing ink to the desired image area and prevent it from transferring to the nonimage areas.
The printing apparatus commonly used in a lithographic process includes a printing plate which is treated to provide an oleophilic ink-accepting image area and an oleophobic ink-repelling nonimage area. Generally, the oleophilic or oil attracting image areas are hydrophobic or water repelling, and the oleophobic or oil repelling nonimage areas are hydrophilic or water attracting.
During a conventional lithographic printing process, an oil based ink composition and an aqueous fountain solution are applied to the printing plate. Because of the solubility and the wetability differences of the oil based and aqueous compositions, the fountain solution is preferentially attracted to and preferentially wets the oleophobic nonimage areas, while the ink is preferentially attracted to and preferentially wets the oleophilic image areas. It is well known and accepted in the art that successful lithographic printing requires inks which exhibit stability relative to water and thus remain water insoluble. Absence of this characteristic will result in poor print quality, poor edge definition and various other print deficiencies Thus, lithographic inks are selected and formulated for their ability to remain stable, cohesive and insoluble when contacted with water. The simultaneous feeding of the ink composition and fountain solution to the print plate is accomplished through a variety of methods and roller configurations known in the art.
In the course of lithographic printing, the printing plates will be periodically changed as one job is completed and another started. Whenever this occurs, the blanket cylinder in an offset process must be cleaned to remove ink residue which is present from the previous job. Further, if a change of ink is desired, the entire print train including the application rollers, the print plate and the blanket must be cleaned. Such cleaning is commonly accomplished using an appropriately formulated wash solvent. To be effective as a wash solvent, the wash must be compatible with, or be able to dissolve, the ink. Since the inks are oil based and have been formulated to be insoluble in water, this normally requires the use of organic or petroleum based wash solvents to effectively remove the ink from the rollers, printing plates, blanket cylinder, etc.
These organic wash solvents can give rise to employee safety concerns and are a large source of both air and water pollution as volatile organic compounds (VOC's) are dispersed into the atmosphere or disposal systems. Such pollution is due to evaporation into the ambient air or into a venting system during the washing of the print components as well as the disposal or laundering of shop towels and rags used in the cleaning process. Attempts to develop water based washes or cleaning solutions have not been successful due to the inherent ability of the ink to resist water. Attempts have also been made to use water/solvent mixtures by emulsifying petroleum-based solvents into water through the use of emulsifiers and surfactants, but these products suffer from inherent instability due to immiscibility of water and the solvents and perform slowly. Further, such mixtures do not completely eliminate the use of petroleum solvents which are still commonly present in an amount of about 30-80%. Other industry trends include the use of solvents such as terpenes. While not petroleum based, their performance has been marginal and they are costly and in short supply. In addition, their use and disposal also pose environmental concerns.
Some attempts have been made to eliminate VOCs during the clean up of lithographic ink using vegetable oil/emulsifier blends or fatty acids/nonionic surfactant mixtures. See U.S. Pat. No. 5,009,716 and 5,104,567 issued to Gerson and Staehr, respectively. In both applications, relatively large quantities of wash solutions are required to fully clean and flush the equipment. Further, when clean up is completed, many of the same problems of biodegradability, environmental and disposal exist with respect to the wash solution and removed ink composition.
Thus, although offset lithography is recognized and established as a dominant printing process for certain applications, drawbacks exist because of the pollution problems referred to above. These are becoming more of a drawback and more of a problem as new pollution control regulations and standards are mandated. In fact, because of the very nature of the lithographic process at least one of the components (either the ink composition or the fountain solution) must be oil based. Thus, the clean up of the lithographic apparatus normally requires the use of an organic or petroleum based solvent or other compositions posing biodegradability environmental or disposal problems. This is generally accepted as a necessary limitation of the lithographic process about which little can be done. Accordingly, there is a need in the art to address this particular limitation and to substantially reduce if not eliminate these pollution concerns.